Extensible split data sync (#11930)

* Extensible split data sync capability through transactions.

- Split common transport has been split up between the transport layer
  and data layer.
- Split "transactions" model used, with convergence between I2C and
  serial data definitions.
- Slave matrix "generation count" is used to determine if the full slave
  matrix needs to be retrieved.
- Encoders get the same "generation count" treatment.
- All other blocks of data are synchronised when a change is detected.
- All transmissions have a globally-configurable deadline before a
  transmission is forced (`FORCED_SYNC_THROTTLE_MS`, default 100ms).
- Added atomicity for all core-synced data, preventing partial updates
- Added retries to AVR i2c_master's i2c_start, to minimise the number of
  failed transactions when interrupts are disabled on the slave due to
  atomicity checks.
- Some keyboards have had slight modifications made in order to ensure
  that they still build due to firmware size restrictions.

* Fixup LED_MATRIX compile.

* Parameterise ERROR_DISCONNECT_COUNT.

keyboards/helix/rev2/keymaps/default/rules.mk

@@ -5,7 +5,7 @@
 #   See TOP/keyboards/helix/rules.mk for a list of options that can be set.
 #   See TOP/docs/config_options.md for more information.
 #
-LTO_ENABLE = no  # if firmware size over limit, try this option
+LTO_ENABLE = yes  # if firmware size over limit, try this option
 
 # Helix Spacific Build Options
 # you can uncomment and edit follows 7 Variables